Apparatus for manufacturing molten metal coated steel strip and method for manufacturing molten metal coated steel strip

ABSTRACT

An apparatus for manufacturing a molten metal coated steel strip configured to control the amount of a metal coated on surfaces of a steel strip by blowing a gas from gas wiping nozzles to the surfaces of the steel strip continuously drawn from a molten metal, includes molten-metal-reducing members arranged on both sides of the steel strip below the surface of the molten metal in the coating bath to face the steel strip, each of the molten-metal-reducing members having a length equal to or longer than the steel-strip width, and shields arranged between the molten-metal-reducing members each extending along an extension of a corresponding one of the surfaces of the steel strip.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2008/068134, withan international filing date of Sep. 30, 2008 (WO 2009/048031 A1,published Apr. 16, 2009), which is based on Japanese Patent ApplicationNo. 2007-262855, filed Oct. 9, 2007, the subject matter of which isincorporated by reference.

TECHNICAL FIELD

This disclosure relates to an apparatus for manufacturing a molten metalcoated steel strip in a molten metal coating process, the apparatusbeing configured to reduce splashing of the molten metal, and a methodfor manufacturing a molten metal coated steel strip with the apparatus.

BACKGROUND

A typical apparatus and process for continuous molten metal coating isdescribed with reference to FIG. 7. A gas wiping device is arranged, thegas wiping device being configured to control the amount of a moltenmetal (coating weight) coated on a steel strip S by blowing apressurized gas from gas wiping nozzles 3 to the steel strip to removean excess amount of the molten metal, the gas wiping nozzles 3 extendingin the direction of the width of the steel strip and being arranged onboth sides of the steel strip S to face the steel strip S such that themolten metal sticking to the surface of the steel strip uniformly has apredetermined thickness in the transverse and longitudinal directions,the blowing step being performed after the steps of immersing the steelstrip S in the molten metal 8 contained in a coating bath 9, changingthe travel direction using a sink roll 7, and drawing the steel strip Sin the vertical direction.

Submersed support rolls 5 are usually arranged above the sink roll 7 andbelow the molten metal surface to stabilize the travel position of thesteel strip in the gas wiping portion. Support rolls 4 outside the bathare arranged above the gas wiping nozzles 3, as needed, in the case ofperforming alloying treatment.

The gas wiping nozzles 3 are usually longer than the width of the steelstrip, i.e., each extend beyond the ends of the steel strip S in thewidth direction so that steel strips correspond with various widths andthe displacement in the width direction in drawing the steel strip. Inthe case of using such a gas wiping device, splashing, in which themolten metal dropping toward the lower portion of the steel strip isspattered due to the turbulence of a jet impinging on the steel strip Soccurs, leading to a reduction in the surface quality of the steelstrip.

The threading speed may be increased to increase the volume ofproduction in the continuous process. In the case where in thecontinuous molten metal coating process, the coating weight iscontrolled by the gas wiping method, the initial amount of the moltenmetal applied to the steel strip immediately after the steel strippasses through the molten metal is increased with increasing line speeddue to the viscosity of the molten metal. Thus, the wiping gas pressureis forced to be set at a higher level to control the coating weightwithin a certain range. This results in a significant increase in theamount of splash, thereby reducing the surface quality.

To overcome the foregoing problems, a method for reducing an excessamount of molten metal sticking to a steel strip to some extent beforethe steel strip reaches the wiping nozzles to reduce the initial amountof the molten metal sticking to the steel strip immediately after thesteel strip passes through the molten metal is disclosed.

Japanese Unexamined Patent Application Publication No. 2004-76082discloses an apparatus including molten-metal-reducing members arrangedon both sides of a steel strip to face the steel strip in a noncontactmanner and arranged between submersed support rolls 5 and wiping nozzles3 in a molten metal, in which an excess amount of molten metal isremoved, and then gas wiping is performed to control the coatingthickness. Each of the molten-metal-reducing members preferably has arectangular shape, a shape having an entry portion in which the distancebetween the member and the corresponding surface of the steel stripincreases with decreasing distance from the lower end of the member, ora columnar shape. JP '082 states that the molten-metal-reducing membersare most preferably located to cross the surface of the molten metal. Italso states that the molten-metal-reducing members are preferablyarranged to surround the steel strip.

In this method, the molten-metal-reducing members can reduce an excessamount of the molten metal sticking to the middle portion of the steelstrip in the width direction. However, the flow of the molten metal fromthe outside of both ends of the steel strip in the width direction intothe middle portion of the steel strip in the width direction reduces theeffect of reducing the molten metal at both ends of the steel strip inthe width direction, thus increasing the difference in excess amount ofmolten metal between the middle portion and both ends in the widthdirection compared with the case where the molten-metal-reducing membersare not arranged. This reduces the effect of reducing splashing in thesubsequent gas-wiping step. Furthermore, the molten-metal-reducingmembers arranged to surround the steel strip as disclosed in that methodcannot correspond a change in the width of a steel strip produced andthus limits the strip width such that the effect of reducing the moltenmetal can be provided.

In consideration of the foregoing problems, it would therefore behelpful to provide an apparatus for stably manufacturing a molten metalcoated steel strip having excellent surface appearance, in which anexcess amount of molten metal sticking to a steel strip drawn from amolten metal can be reduced across the entire width of the steel stripeven when the width of the steel strip is changed, so that theoccurrence of splashing can be suppressed in a gas-wiping step.

It could also be helpful to provide a method for stably manufacturing amolten metal coated steel strip having excellent surface appearance inwhich the occurrence of splashing can be suppressed in the gas-wipingstep.

SUMMARY

We provide (1) an apparatus for manufacturing a molten metal coatedsteel strip, the apparatus being configured to control the amount of ametal coated on surfaces of a steel strip by blowing a gas from gaswiping nozzles to the surfaces of the steel strip continuously drawnfrom a molten metal, including molten-metal-reducing members arranged onboth sides of the steel strip below the surface of the molten metal inthe coating bath to face the steel strip, each of themolten-metal-reducing members having a length equal to or longer thanthe width of the steel strip, and shields arranged between themolten-metal-reducing members each extending along an extension of acorresponding one of the surfaces of the steel strip, themolten-metal-reducing members being arranged to face the steel strip.

Furthermore, (2) in the apparatus for manufacturing a molten metalcoated steel strip described in item (1), the length of a surface ofeach of the shields facing the steel strip in the travel direction ofthe steel strip is 50% or more of the length of a steel-strip-facingsurface of each of the molten-metal-reducing members in the traveldirection of the steel strip (when the steel-strip-facing surfaces ofthe molten-metal-reducing members in the travel direction of the steelstrip have different lengths on both sides of the steel strip, thelength of the surface of each of the shields facing the steel strip inthe travel direction of the steel strip is 50% or more of the length ofthe steel-strip-facing surface of the molten-metal-reducing memberhaving a shorter length of the steel-strip-facing surface in the traveldirection of the steel strip), and the distance between themolten-metal-reducing members and the shields is 3 mm or less.

We also provided (3) a method for manufacturing a molten metal coatedsteel strip including coating a steel strip with a molten metal usingthe apparatus for manufacturing a molten metal coated steel stripdescribed in item (1) or (2).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an apparatus for manufacturing amolten metal coated steel strip.

FIGS. 2( a) and 2(b) illustrate the operation of molten-metal-reducingmembers and a shield of an apparatus for manufacturing a molten metalcoated steel strip.

FIG. 3 is a first drawing illustrating an exemplary combination ofcross-sectional shapes of molten-metal-reducing members and a shieldused in an apparatus for manufacturing a molten metal coated steelstrip.

FIGS. 4( a) and 4(b) are second drawings illustrating exemplarycombinations of cross-sectional shapes of molten-metal-reducing membersand the cross-sectional shape of a shield arranged in an apparatus formanufacturing a molten metal coated steel strip.

FIGS. 5( a) and 5(b) are third drawings illustrating exemplarycombinations of cross-sectional shapes of molten-metal-reducing membersand the cross-sectional shape of a shield arranged in an apparatus formanufacturing a molten metal coated steel strip.

FIGS. 6( a) and 6(b) are fourth drawings illustrating exemplarycombinations of cross-sectional shapes of molten-metal-reducing membersand the cross-sectional shape of a shield arranged in an apparatus formanufacturing a molten metal coated steel strip.

FIG. 7 is a cross-sectional view of a known apparatus for manufacturinga molten metal coated steel strip.

DETAILED DESCRIPTION

In the case where the molten-metal-reducing members for removing anexcess amount of molten metal are arranged between a sink roll and gaswiping nozzles, the molten metal removed by gas wiping flows down toform pools between the steel strip and the molten-metal-reducingmembers. If the distance between the pools and the gas wiping portion isshort, an excess amount of molten metal cannot be reduced. We discoveredthat the best position of the molten-metal-reducing member arranged isbelow the molten metal surface.

However, merely arranging the molten-metal-reducing members leads to asmall effect of reducing the molten metal at both ends of the steelstrip. To effectively reduce the amount of the molten metal sticking tothe steel strip drawn from the molten metal, flow analysis was made indetail with a model apparatus for simulating flows of the molten metalaround the molten-metal-reducing member using water. The analysis showedthat suppression of flows from both ends to the middle portion of thesteel strip is effective in reducing the amount of molten metal stickingto the steel strip.

Representative examples of our apparatus and methods will be describedbelow with reference to the attached drawings. In the following figures,elements having the same functions as the elements shown in theexplained figure are designated using the same reference numerals, andredundant descriptions are not made.

FIG. 1 is a cross-sectional view of an apparatus for manufacturing amolten metal coated steel strip. Reference numeral 1 denotesmolten-metal-reducing members arranged in a molten metal, above thesubmersed support rolls 5, and on both sides of the steel strip S, eachof the molten-metal-reducing members being disposed a predetermineddistance apart from a corresponding one of the surfaces of the steelstrip. Reference numeral 2 denotes shields arranged close to both endsof the steel strip S and between the molten-metal-reducing members 1which each extend along an extension of a corresponding one of thesurfaces of the steel strip and which are located to face the steelstrip S. The phrase “along an extension of a corresponding one of thesurfaces of the steel strip” indicates “along a line parallel to thewidth direction of the steel strip.” The term “shields” indicates amember configured to shield the molten metal and inhibit flows from bothends to the middle portion of the steel strip.

FIGS. 2( a) and 2(b) illustrate operation of the molten-metal-reducingmembers and the shields of the apparatus. FIG. 2( a) is a top viewillustrating flows of a molten metal at an end of a steel strip in aregion between the molten-metal-reducing members when only themolten-metal-reducing members are arranged. FIG. 2( b) is a top viewillustrating flows of a molten metal at an end of a steel strip in aregion between the molten-metal-reducing members when themolten-metal-reducing members and the shields 2 are arranged. Even inthe case where the molten-metal-reducing members 1 has any shape, thearrangement of only the molten-metal-reducing members 1 produces flows11 of the molten metal from the ends to the middle portion of the steelstrip as shown in FIG. 2( a). A larger molten-metal-reducing effect ofthe molten-metal-reducing members 1 is liable to cause an increase inthe flows 11 as if the flows 11 compensate for the effect, therebydisadvantageously reducing or eliminating the reduction effect of themolten-metal-reducing members 1 at both ends of the steel strip. In thecase where the shields 2 are arranged between the molten-metal-reducingmembers which each extend along an extension of a corresponding one ofthe surfaces of the steel strip and which are located to face the steelstrip as shown in FIG. 2( b), the flows of the molten metal from theends to the middle portion of the steel strip can be shielded. Hence,the effect of reducing an excess amount of molten metal by themolten-metal-reducing members 1 can be uniformly provided across theentire width of the steel strip.

The thickness of the molten metal can be controlled by the gas wipingnozzles after an excess amount of molten metal sticking to the steelstrip is reduced across the entire width of the steel strip using themolten-metal-reducing members and the shields even when the width of thesteel strip is changed, thereby significantly reducing the amount ofsplashing. The effect of reducing the molten metal can be provided evenat a significantly increased threading speed, thereby significantlyreducing the amount of splashing. It is thus possible to manufacture asurface defect-free molten metal coated steel strip with highproductivity.

An end face of each shield 2 adjacent to the steel strip is desirablyperpendicular to the steel strip surfaces as shown in FIG. 2( b). Thedistance between each end of the steel strip and a corresponding one ofthe end faces of each shield 2 adjacent to the steel strip is desirably5 mm or less. A smaller distance therebetween is more desirable. As mostsuitable conditions, the ends of the steel strip are in contact with theend faces of the shields 2 while no pressing force is being applied tothe steel strip.

The clearance between the molten-metal-reducing members 1 and theshields 2 is desirably 3 mm or less. A smaller clearance therebetween ismore desirable.

From the viewpoint of preventing the flows of the molten metal towardthe middle portion of the steel strip between the molten-metal-reducingmembers 1, the length of a surface of each shield 2 facing the steelstrip in the travel direction of the steel strip (the length in thevertical direction) is preferably at least 50% or more of the length ofthe molten-metal-reducing members 1 in the travel direction of the steelstrip and most preferably a length comparable to the length of themolten-metal-reducing members 1.

In the case where the distance between the steel strip and the surfaceof each molten-metal-reducing member 1 facing the steel strip is changedin the travel direction of the steel strip, the clearance betweenmolten-metal-reducing members 1 and the shields 2 is preferablymaintained constant. For example, when the molten-metal-reducing members1 each have a circular cross-section as shown in FIG. 3, each of theshields 2 preferably has concave arcuate surfaces facing themolten-metal-reducing members 1, each of the concave arcuate surfaceshaving a radius of curvature slightly larger than that of the circulararc of each molten-metal-reducing member 1.

The cross-sectional shape of each of the molten-metal-reducing membersis not limited to the shape shown in FIG. 3. The molten-metal-reducingmembers may have various cross-sectional shapes as described below. Forexample, as shown in FIG. 4( a), in the case where each of themolten-metal-reducing members 1 has a triangular cross-section, asurface facing the steel strip S, and the top surface facing themolten-metal surface, the surface facing the steel strip S beingparallel to the steel strip S, and the top surface being parallel to themolten-metal surface, the molten-metal-reducing members 1 can provide animproved effect of reducing the molten metal. In the case of using themolten-metal-reducing members 1 each having the shape, even if the flow(accompanying flow) 11 associated with the travel of the steel strip Sis generated, the flow 11 branches off at the bottom of themolten-metal-reducing members 1 to form a flow 13 because fluid flowseasily in the low-resistance direction. Hence, the molten-metal-reducingmembers 1 have the function to prevent the growth of the flow 11.Furthermore, the flow 13 flows in the direction away from the steelstrip S and thus faces a flow 12 flowing toward the steel strip S abovethe molten-metal-reducing members 1. Hence, the molten-metal-reducingmembers 1 also have the effect of reducing the velocity of the flow 12.The molten-metal-reducing members 1 can control the flow as describedabove and thus can significantly suppress accompanying flows near thesteel strip S drawn from the molten metal, thereby reducing an excessamount of molten metal sticking to the steel strip S. As a result, thepressure of a wiping gas can be lowered to reduce the amount ofsplashing of the molten metal so that a steel strip having satisfactorysurface quality can be produced. In this case, each of the shields 2 mayhave a rectangular cross-section as shown in FIG. 4( b).

Each of the molten-metal-reducing members 1 shown in FIG. 5( a) has across-sectional shape in which an upper profile curve and a lowerprofile curve are both arcuate and convex in the direction in which thesteel strip is drawn from the molten metal and in which the upper curvehas a radius of curvature smaller than that of the lower curve.Furthermore, the thickness of each of the molten-metal-reducing members1 decreases with increasing distance from the steel strip and from themolten-metal surface. This shape of each molten-metal-reducing member 1most notably provides the effect of branching the flow 11 to form theflow 13 and the effect of forming a flow opposite the flow 12.

In this case, as shown in FIG. 5( b), each of the shields 2 preferablyhas concave arcuate surfaces facing the molten-metal-reducing members 1,each of the concave arcuate surfaces having a radius of curvatureslightly larger than that of the circular arc of a surface of eachmolten-metal-reducing member 1 facing the shields 2, and the distancebetween each molten-metal-reducing member 1 and the corresponding shield2 is preferably maintained constant.

Molten-metal-reducing members 1 a and 1 b shown in FIG. 6( a) each havea roll-covering portion and a steel-strip-facing portion, each of theroll-covering portions being configured to cover the periphery of acorresponding one of the submersed support rolls 5 near to the moltenmetal surface, and each of the steel-strip-facing portions beingarranged above a corresponding one of the steel-strip-facing portionsand facing the steel strip. The submersed support rolls 5 are arrangedon both sides of the steel strip to be in contact with the steel strip,the submersed support rolls 5 being located at different positions inthe vertical direction. Thus, the steel-strip-facing portions of themolten-metal-reducing members 1 a and 1 b arranged on both sides of thesteel strip S have different lengths in the travel direction of thesteel strip. Each of the steel-strip-facing portions of themolten-metal-reducing members 1 a and 1 b may be parallel or oblique tothe surfaces of the steel strip.

The arrangement of the molten-metal-reducing members 1 a and 1 b resultsin the generation of flows 14 due to the submersed support rolls 5between the submersed support rolls 5 and the molten-metal-reducingmembers 1 a and 1 b. The generation of the flows 14 results in thegeneration of forced flows 16 between the steel strip S and themolten-metal-reducing members 1 a and 1 b, the forced flows 16 flowingin the direction opposite to the travel direction of the steel strip S,even when accompanying flows 15 are generated by the travel of the steelstrip S, thereby significantly suppressing the accompanying flows 15.This results in a reduction in an excess amount of molten metal stickingto the steel strip S drawn from the molten metal.

The molten-metal-reducing members may have only the steel-strip-facingportions of the molten-metal-reducing members 1 a and 1 b shown in FIG.6( a). In this case, the steel-strip-facing portions of themolten-metal-reducing members 1 a and 1 b arranged on both sides of thesteel strip may have the same length.

In the case of the molten-metal-reducing members described above orshown in FIG. 6( a), each of the shields 2 may have a rectangular crosssection as shown in FIG. 6( b). In this case, the length of the surfaceof each shield 2 facing the steel strip in the travel direction of thesteel strip is preferably at least 50% or more of the length of thesteel-strip-facing surfaces of the molten-metal-reducing members in thetravel direction of the steel strip (when the steel-strip-facingsurfaces of the molten-metal-reducing members in the travel direction ofthe steel strip have different lengths on both sides of the steel strip,the length of the surface of each shield 2 facing the steel strip in thetravel direction of the steel strip is preferably 50% or more of thelength of the steel-strip-facing surface of the molten-metal-reducingmember having a shorter length of the steel-strip-facing surface in thetravel direction of the steel strip). More preferably, the length of thesurface of each shield 2 facing the steel strip in the travel directionof the steel strip is comparable to the length of the steel-strip-facingsurfaces of the molten-metal-reducing members in the travel direction ofthe steel strip (when the steel-strip-facing surfaces of themolten-metal-reducing members in the travel direction of the steel striphave different lengths on both sides of the steel strip, the length ofthe surface of each shield 2 facing the steel strip in the traveldirection of the steel strip is more preferably comparable to the lengthof the steel-strip-facing surface of the molten-metal-reducing memberhaving a shorter length of the steel-strip-facing surface in the traveldirection of the steel strip).

The shape and dimensions of the molten-metal-reducing members need to beappropriately determined in view of the threading speed, facility, andthe like.

The height position of the shields 2 in the travel direction of thesteel strip is preferably comparable to that of themolten-metal-reducing members 1. The upper and lower ends of each shield2 are preferably located at the same vertical positions as those of theupper and lower ends of the molten-metal-reducing members 1. In the casewhere the length of shields 2 in the travel direction of the steel stripis shorter than the length of the molten-metal-reducing members 1 in thetravel direction of the steel strip, the shields 2 are preferablyarranged near to the molten-metal surface, i.e., the shields 2 arepreferably arranged in such a manner that the upper end of each shield 2is located at substantially the same position as the upper ends of themolten-metal-reducing members 1. Each of the shields 2 preferably has alength of 100 mm or more in the width direction of the steel strip.Although the upper limit thereof is not limited, the length ispreferably about 500 mm or less because a larger length requires alarger facility.

EXAMPLES

The apparatus for manufacturing a molten metal coated steel strip asshown in FIG. 1 was installed in a continuous hot-dip galvanizing line.An experiment for producing a hot-dip galvanized steel strip wasperformed. The amount of offset of the submersed support rolls arrangedon both sides of the steel strip S was 200 mm in the vertical direction.The distance between the molten-metal surface and the top of thesubmersed support roll closer to the molten metal surface was 80 mm.Each of the submersed support rolls had a diameter of 400 mm.

The length of the molten-metal-reducing members 1 in the width directionof the steel strip was 2,000 mm which was comparable to that of the gaswiping nozzles. The molten-metal-reducing members 1 were securelyarranged on both sides of the steel strip to face the surfaces of thesteel strip and in such a manner that the distance between the upperends of the molten-metal-reducing members and the molten-metal surfacewas 5 mm and that the distance between the molten-metal-reducing members1 and the steel strip was 3 mm (excluding Comparative Example 5 andExample 4). The shields 2 had a length of 200 mm in the width directionof the steel strip. The shields 2 were directly coupled to framesextending from position controllers with servomotors arranged on theoutside and were thus movable in response to the width of the steelstrip.

Conditions for manufacturing the hot-dip galvanized steel strip were asfollows: the slit gap of each gas wiping nozzle: 0.8 mm, gas wipingnozzle-steel strip distance: 7 mm, nozzle height from the molten zincsurface: 400 mm, and the temperature of the molten zinc bath: 460° C.The steel strip to be manufactured had a thickness of 0.8 mm, a width of1.2 m, and a coating weight of 45 g/m² per side. The distance betweenthe shields 2 and the ends of the steel strip was about 3 mm.

Table 1 shows other manufacturing conditions and the amount of splashingserving as a product quality index. Shapes and dimensions of themolten-metal-reducing members and the shields used in the comparativeexamples and examples will be described below. The amount of splashingis defined as the ratio of the length of the steel strip determined as astrip having splash defects in an inspection step to the length of thesteel strip fed under such production conditions. The resulting steelstrips contained practically negligible splash defects.

TABLE 1 Presence or absence and Operation conditions cross-sectionalshape Presence or absence Wiping Threading of molten-metal- andcross-sectional Amount of pressure speed reducing member shape of shieldsplashing Example 1 0.6 kgf/cm² 2.5 m/sec Present (square) Present(rectangle) 0.72% Example 2 0.6 kgf/cm² 2.5 m/sec Present (triangle,[FIG. 4(a)]) Present ([FIG. 4(b)]) 0.28% Example 3 0.6 kgf/cm² 2.5 m/secPresent (arcuate, [FIG. 5(a)]) Present ([FIG. 5(b)]) 0.16% Example 4 0.6kgf/cm² 2.5 m/sec Present (with arcuate roll-covering Present ([FIG.6(b)]) 0.09% portion and steel-strip-facing portion parallel to steelstrip surface, [FIG. 6(a)]) Comparative 0.6 kgf/cm² 2.5 m/sec AbsentAbsent 1.40% Example 1 Comparative 0.6 kgf/cm² 2.5 m/sec Present(square) Absent 1.05% Example 2 Comparative 0.6 kgf/cm² 2.5 m/secPresent (triangle, [FIG. 4(a)]) Absent 0.41% Example 3 Comparative 0.6kgf/cm² 2.5 m/sec Present (arcuate, [FIG. 5(a)]) Absent 0.23% Example 4Comparative 0.6 kgf/cm² 2.5 m/sec Present (with arcuate roll-coveringAbsent 0.21% Example 5 portion and steel-strip-facing portion parallelto steel strip surface, [FIG. 6(a)])

In Comparative Example 1 (related example), none of themolten-metal-reducing members and the shields were arranged. The splashrate was 1.40%.

In Comparative Example 2, only molten-metal-reducing members each havinga square cross-section with a length in the travel direction of thesteel strip of 50 mm and a length of in the horizontal direction of 50mm were used. In Example 1, shields each having a rectangularcross-section with a length in the travel direction of the steel stripof 50 mm and a length in the horizontal direction of 4 mm were arrangedin addition to the structure in Comparative Example 2 (the distancebetween the molten-metal-reducing members and the shields was 1 mm).

In Comparative Example 2, the splash rate was reduced by about 25% withrespect to that in Comparative Example 1. In Example 1, the splash ratewas reduced by almost half with respect to that in Comparative Example 1and by about 31% with respect to Comparative Example 2.

In Comparative Example 3, only molten-metal-reducing members each havinga triangular cross-section with a length in the travel direction of thesteel strip of 50 mm and a length of in the horizontal direction of 50mm were arranged as shown in FIG. 4( a). In Example 2, shields eachhaving a rectangular cross-section with dimensions the same as inExample 1 were arranged as shown in FIG. 4( b) in addition to thestructure in Comparative Example 3 (the distance between themolten-metal-reducing members and the shields was 1 mm). In ComparativeExample 3, the splash rate was reduced by about 70% with respect to thatin Comparative Example 1. In Comparative Example 2, the splash rate wasreduced by about 80% with respect to that in Comparative Example 1 andby about 32% with respect to that in Comparative Example 3.

In Comparative Example 4, only molten-metal-reducing members each havingan arcuate cross-section with a length in the travel direction of thesteel strip of 50 mm and a length of in the horizontal direction of 50mm (the upper curve had a radius of curvature of 60 mmR, and the lowercurve had a radius of curvature of 100 mmR) were arranged as shown inFIG. 5( a). The distance between bottoms of the molten-metal-reducingmembers and the steel strip was 3 mm.

In Example 3, shields each having a cross-sectional shape shown in FIG.5( b), i.e., shields each having concave arcuate surfaces facing themolten-metal-reducing members were arranged as shown in FIG. 5( b) inaddition to the structure in Comparative Example 4, each of the shieldhaving a length in the travel direction of the steel strip of 50 mm,each of the concave arcuate surfaces having a radius of curvatureslightly larger than that of the circular arc of a surface of eachmolten-metal-reducing member facing the shields, and the distancebetween the shields and the molten-metal-reducing members being 1 mm.

In Comparative Example 4, the splash rate was reduced by about 84% withrespect to that in Comparative Example 1. In Example 3, the splash ratewas reduced by about 90% with respect to that in Comparative Example 1and by about 30% with respect to that in Comparative Example 4.

In Comparative Example 5, only molten-metal-reducing members 1 a and 1 beach having an arcuate roll-covering portion and a platesteel-strip-facing portion were arranged as shown in FIG. 6( a), each ofthe arcuate roll-covering portions covering the periphery of acorresponding one of the submersed support rolls 5 near to the moltenmetal surface and being arranged in such a manner that the distancebetween the submersed support rolls 5 and the molten-metal-reducingmembers 1 a and 1 b is 30 mm, and each of the plate steel-strip-facingportions being arranged in such a manner that the distance between thesteel strip and the molten-metal-reducing members 1 a and 1 b is fixedto 20 mm and the upper end of the molten-metal-reducing members 1 a and1 b and the molten-metal surface is 30 mm.

In Example 4, shields each having a length in the travel direction ofthe steel strip of 100 mm and a length in the horizontal direction of 36mm were arranged as shown in FIG. 6( b) in addition to the structure inComparative Example 5. The distance between the shields and themolten-metal-reducing members was 2 mm. The rate of the length of eachshield in the travel direction of the steel strip to the length of thesteel-strip-facing portion of the molten-metal-reducing members 1 b inthe travel direction of the steel strip was about 90%.

In Comparative Example 5, the splash rate was reduced by about 85% withrespect to that in Comparative Example 1. In Example 4, the splash ratewas reduced by about 94% with respect to that in Comparative Example 1and by about 33% with respect to that in Comparative Example 5.

As described above, the thickness of the coating metal can be controlledby the gas wiping nozzles after an excess amount of molten metalsticking to the steel strip is reduced across the entire width of thesteel strip using the molten-metal-reducing members and the shieldsarranged below the molten-metal surface even when the width of the steelstrip is changed, thereby significantly reducing the amount ofsplashing. Furthermore, the amount of splashing can be significantlyreduced even at a significantly increased threading speed, therebymanufacturing a surface defect-free molten metal coated steel strip withhigh productivity.

INDUSTRIAL APPLICABILITY

Our apparatus suppresses the occurrence of splashing and thus can beused as an apparatus for manufacturing a molten metal coated steel striphaving excellent surface appearance. The apparatus can suppress theoccurrence of splashing even at high-speed threading and thus can beused as an apparatus for manufacturing a molten metal coated steel striphaving excellent surface appearance with high productivity. Furthermore,the method for manufacturing a steel strip can be used as a method formanufacturing a molten metal coated steel strip having excellent surfaceappearance by reducing the occurrence of splashing.

1. An apparatus for manufacturing a molten metal coated steel stripconfigured to control the amount of metal coated on surfaces of a steelstrip by blowing a gas from gas wiping nozzles to the surfaces of thesteel strip continuously drawn from a molten metal comprising;molten-metal-reducing members arranged on both sides of the steel stripbelow the surface of the molten metal in the coating bath to face thesteel strip, each of the molten-metal-reducing members having a lengthequal to or longer than the width of the steel strip, and shieldsarranged between the molten-metal-reducing members and extending alongan extension of a corresponding one of the surfaces of the steel strip.2. The apparatus according to claim 1, wherein the length of a surfaceof each of the shields facing the steel strip in the travel direction ofthe steel strip is 50% or more of the length of a steel-strip-facingsurface of each of the molten-metal-reducing members in the traveldirection of the steel strip (when the steel-strip-facing surfaces ofthe molten-metal-reducing members in the travel direction of the steelstrip have different lengths on both sides of the steel strip, thelength of the surface of each of the shields facing the steel strip inthe travel direction of the steel strip is 50% or more of the length ofthe steel-strip-facing surface of the molten-metal-reducing memberhaving a shorter length of the steel-strip-facing surface in the traveldirection of the steel strip), and wherein the distance between themolten-metal-reducing members and the shields is 3 mm or less.
 3. Amethod for manufacturing a molten metal coated steel strip comprisingcoating a steel strip with a molten metal with the apparatus accordingto claim
 1. 4. A method for manufacturing a molten metal coated steelstrip comprising coating a steel strip with a molten metal with theapparatus according to claim 2.